The present invention relates to sensors for detecting unwanted flames within a designated area by monitoring for characteristic infrared radiation emitted by such flames.
Flame detection sensors are known which monitor a region for the characteristic infrared radiation emitted by a flame, the detection of such radiation being taken to be an indication that a flame is present and that a fire alarm should be signaled. However, interfering or false alarm radiation sources, for example halogen lamps, reflected sunlight, discharge lamps, electric welders, hot pipes etc., are often also present in a monitored region which can lead to a sensor incorrectly detecting the presence of a flame.
Existing infrared flame detectors use a variety of different technologies to gather as much information as possible about the radiation emitted within a target area within limits set by cost, complexity, reliability and size. A typical infra-red flame detector known in the art monitors for radiation emitted by hot carbon dioxide within a narrow wavelength band around a wavelength of 4.3 p.un and compares this to the radiation at a nearby wavelength, e.g. 5. Spm. For flames, this spectral ratio of the radiation intensity at 4.3 pm to the radiation intensity at 5.5 pm will be much higher than would be the case for radiation emitted by any other source at the same temperature as a flame, and this alone gives a good indication of the presence or absence of a flame, This system may be complemented by analysis of the flicker frequencies in the signal or by examining the correlation of the signals at the two wavelengths. The control system for such an arrangement is typically programmed with a preset threshold value for the ratio of (4.3 um Intensity)/(5.5 urn Intensity), and if that value is exceeded for a preset time, then an alarm will be activated.
This basic system has the problem, however, that it fails to correctly identify a fire situation in certain circumstances. In particular, if an intense false alarm radiation source is present as well as a flame then the 4.3 um/5.5 um intensity ratio will be dominated by the most intense source present and the relatively low flux of the 4.3 um radiation from a distant or weak flame will not be registered with sufficient accuracy, resulting in the system inferring from the value of the intensity ratio that no flame is present when, in fact, there is. Furthermore, some false alarm sources can generate a spectral output that is very flame-like. This can happen if the source has emitting parts at very different temperatures as in a convector/radiator electric fire or if the source is not a true black body as with mercury or sodium discharge lamps (such sources emit radiation over a large number of narrow wavebands).
in order to overcome this problem, use of an array based infrared detector has been proposed in which an image of die protected area is focused onto a focal plane array. Such a device, when combined with appropriate signal processing, can allow estimates of the angular size of one or more emitting objects and analyze their internal structure and movement for flame-like features. Unfortunately, it is not always possible to make an unambiguous decision on whether certain objects are flames without the spectral information described above. The use of two such arrays overcomes this problem since it will then be possible to calculate a value of the ratio I (4.3 um)/I(5.5 um) for each separately focused object. This solution is, however, very expensive due to the requirement for two high-resolution sensor arrays and may be too costly for many applications. Other systems have been proposed which use mechanical scanning arrangements, but these have the drawback that observation times are reduced and fast events may be missed. Furthermore, in certain application areas there may be considerable customer aversion to the use of moving parts in an apparatus.
According to the present invention there is provided a flame detection apparatus comprising means for generating an image of the infra-red radiation emitted within a viewing region, means for measuring the spectral ratio of the intensity of radiation having a first wavelength emitted within the viewing region to the intensity of radiation having a second wavelength emitted within the region, and processing means which analyses the outputs of said image generating and spectral ratio measuring means for responses indicative of the presence of a flame.
A flame detection apparatus in accordance with the invention has the advantage that it enables particularly accurate and reliable detection of a flame in a monitored region even in the presence of interfering or false alarm radiation sources.
The means for generating an image of the infrared radiation emitted within the viewing area is a preferably focused array based sensor responsive to radiation having a predefined wavelength, preferably in the range 2 to 15 um. The term an array used in this document refers to a two dimensional array, which might typically comprises a 16 by 16 grid of sensors, which is able to generate a two dimensional image of a viewing field. Furthermore, the means for measuring the spectral ratio includes at least one unfocussed volumetric sensor which measures the radiation emitted within the region having one of said first and second wavelengths. This has the advantage that, since the system only requires a single focused array sensor, it is much cheaper than prior art systems of comparable accuracy and reliability.
In one embodiment of the invention, the array sensor is sensitive to one of the first and second wavelengths, preferably the first wavelength which is 4.3 um, and the volumetric sensor is responsive to the other of the first and second wavelengths, preferably the second wavelength which is 5.5 um the processing means summing the total radiation incident on the array based sensor and comparing it with the output of the volumetric sensor in order to calculate the spectral ratio. This has the advantage of reducing the number of components in the system and hence its complexity and cost. Preferably, however, the system includes two volumetric sensors, one that operates at the first wavelength and the other at the second wavelength, the output of the two volumetric sensors being used to calculate the spectral ratio. The array sensor is then dedicated to generating an image of the viewed region. This has the advantage of reducing the complexity of the processing means required to operate the system.
Preferably, the first wavelength is 4.3 um and the second wavelength is 5.5 um, there being a well defined threshold value for the spectral ratio resulting there from which, if exceeded, provides a strong indication of the presence of a hydrocarbon flame. Alternatively, however, other wavelengths could be used, for example 2.9 um instead of 4.3 um, in order to enable other types of flame, in particular non-hydrocarbon flames to be detected.
The operation of the system may be further improved by provision of a second focused array based sensor responsive to radiation having a wavelength which is different from that of said first focused array based sensor. Also, a further unfocussed volumetric sensor may be used which measures the intensity of short wavelength or visible radiation. This has the advantage of further reducing the instances of false alarms being sounded by the system due to, for example, direct sunlight blinding the system. Furthermore, at least one further sensor which measures at least one of: the actual temperature, the rate of rise of temperature and the vibration within the monitored area may also being included in the system, which further information may be utilized by the processing means as a further confirmation of the presence or absence of a fire within the viewing area.
The present invention further provides a method of detecting a flame comprising the steps of measuring the intensity of radiation having a first wavelength within a monitored region, measuring the intensity of radiation having a second wavelength within the monitored region, calculating the spectral ratio of the intensity of the radiation having the first wavelength to the intensity of the radiation having the second wavelength and comparing it to a predefined threshold value indicative of the presence of a flame, generating an image of the infra-red radiation within the monitored region, analyzing the image for features indicative of the presence of a flame within the monitored region, and activating an alarm if the results of the spectral ratio analysis and the image analysis fit a predefined profile indicative of the presence of a flame.
Preferably the first wavelength is 4.3 um and the second wavelength is 3.5 um, particularly effect detection of hydrocarbon fires thereby being possible. However, other wavelengths may also be used in order to detect other types of fires, such as non-hydrocarbon fires, in particular 2.9 um.
In the preferred embodiment, the analysis includes the steps of discerning the number of separate dynamic radiation sources present in the viewing area and analyzing at least one of the shape, movement and intensity of each source for predefined flame-like qualities.
According to an advantageous development of the invention, the method includes the further step of measuring at least one of the actual temperature, the rate of rise of temperature and the vibration within the monitored region, and analyzing the characteristics thereof for behavior indicative of the presence of a flame, by means of which additional information is available to the processor for confirming the presence or absence of a flame. The accuracy and reliability of the system may be still further improved by measuring the intensity of at least one of the short wavelength radiation and the visible radiation within the viewing area and analyzing the profile thereof for characteristics indicative of a non-flame radiation source.